Electrode-supported tubular solid-oxide electrochemical cell

ABSTRACT

Electrode-supported tubular solid-oxide electrochemical cells suitable for use in electrochemical synthesis and processes for manufacturing such are provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 14/878,544, filed on Oct. 8, 2015 and incorporated herein byreference in its entirety.

A portion of the disclosure of this patent document contains materialthat is subject to copyright protection. The copyright owner has noobjection to the facsimile reproduction by anyone of the patent documentor the patent disclosure, as it appears in the Patent and TrademarkOffice patent files or records, but otherwise reserves all copyrightrights whatsoever. The following notice applies to the software and dataas described below and in the drawings that form a part of thisdocument: Copyright Low Emission Resources Corporation, 2015, All RightsReserved.

TECHNICAL FIELD

The present disclosure relates to solid state electrochemical cells,more particularly to electrode-supported tubular solid-oxideelectrochemical cells for use in electrochemical synthesis of chemicalsand methods of making such electrode-supported tubular solid-oxideelectrochemical cells.

BACKGROUND

Electrochemical cells that incorporate ion conducting solid electrolyteshave shown great promise for gaseous chemical synthesis applications.Electrochemical synthesis using such ion conducting solid electrolytescan produce high purity gases at higher reaction rates, with lower costand without several of the chemical by-products and detrimentalenvironmental impacts of traditional catalytic chemical synthesisprocesses.

For example, the traditional catalytic production of hydrogen gas (H₂)and of ammonia (NH₃), and the steps involved in their commercial scaleimplementation, are very energy intensive processes and produce massiveamounts of carbon dioxide (CO₂), a greenhouse gas widely acknowledged bythe global scientific community as contributing to the warming of theearth's atmosphere and oceans.

Hydrogen gas (H₂) is an important starting material for many industrialchemicals and also important as a primary fuel source in renewableenergy production. Currently, most industrial hydrogen gas productioninvolves catalytic steam reforming of a carbonaceous feed, such asnatural gas, coal, liquefied petroleum gas, or the like, as follows:C_(n)H_(m) +nH₂O

(n+m/2)H₂ +nCOCO+H₂O

H₂+CO₂This process typically requires high temperatures (such as 700-1000° C.)and, in practice, involves various additional steps, such as removingsulfur from the carbonaceous feed.

Ammonia (NH₃) is one of the most highly produced inorganic chemicals inthe world because of its many commercial uses, such as in fertilizers,explosives and polymers. Modern commercial production of ammoniatypically utilizes some variation of the Haber-Bosch process. TheHaber-Bosch process involves the reaction of gaseous nitrogen (N₂) andhydrogen (H₂) on an iron-based catalyst at high pressures (such as150-300 bar) and high temperature (such as 400-500° C.), as follows:N₂+3H₂

2NH₃In modern ammonia-producing plants, the nitrogen feed typically derivesfrom atmospheric air but the hydrogen feed typically derives fromcatalytic steam reforming of a carbonaceous feed stock, discussed above.In practice, implementation of this process requires various othersteps, such as separating and purifying the hydrogen before it can beused.

Commercial scale production of industrially important chemicals, such ashydrogen and ammonia, may be achieved more efficiently and costeffectively by electrochemical synthesis than by the traditionalcatalytic processes such as those discussed above. For example, hydrogengas can be produced by direct electrolysis of steam (H₂O) withoutrequiring a carbonaceous feed source and the commensurate production ofcarbon dioxide. Ammonia may be electrochemically produced by directlyreacting the hydrogen with nitrogen, eliminating the need forintermediate steps to separate and purify the hydrogen before use in thesynthesis reaction.

Electrochemical synthesis is typically carried out using anelectrochemical cell that incorporate two electrodes (an anode and acathode) and an electrolyte that separates the two electrodes. As usedin electrochemical synthesis applications, the two electrodes areconnected via electronic circuitry to a power source, and theelectrolyte typically is a material that conducts ionic species but notelectrons nor non-ionized species, such as the initial chemicalreactants and final chemical products. When a voltage is applied acrossthe two electrodes, a reactant is dissociated and ionized at oneelectrode, and the ionized reactant species migrates through theelectrolyte toward the opposite electrode, where it reacts (in somecases with a second reactant that is present at the opposite electrode)to form the desired reaction product. The materials and configuration ofthe electrodes and electrolyte are selected and optimized depending onthe desired electrochemical synthesis reaction.

For electrochemical synthesis to be widely applicable and commerciallyviable, there exists a need for electrochemical cells that may befabricated using a variety of materials and in various configurations,depending on the desired electrochemical synthesis reaction, in acost-effective and scalable way. Moreover, it is desirable for theelectrochemical cells to have the structural and chemical stability anddurability to withstand the potentially severe temperatures, pressuresand chemical environments in which they would operate.

SUMMARY

In one aspect, the present disclosure provides a tubular solid-oxideelectrochemical cell that includes a first porous electrode configuredin a tubular shape, an electrolyte disposed as a thin-layer on at leastpart of a surface of the first porous electrode, and a second porouselectrode disposed on at least a part of a surface of the electrolyte.The first porous electrode includes a first mixedionically/electronically conductive composite material of a solid-oxideelectrolyte substance and a first electrochemically active metallicsubstance. The electrolyte includes the solid-oxide electrolytesubstance. The second porous electrode includes a second mixedionically/electronically conductive composite material of the samesolid-oxide electrolyte substance and a second electrochemically activemetallic substance.

In another aspect, the present disclosure provides a tubular solid-oxideelectrochemical cell made by extruding an electrode dough in a tubularshape to form a first electrode, wherein the electrode dough includes asolid-oxide electrolyte substance, a first electrochemically activemetallic substance, a carbon-based pore forming substance, a binder anda solvent; sintering the extruded first electrode to cause thecarbon-based pore forming substance to burn off and form pores in thefirst electrode; forming an electrolyte onto at least part of a surfaceof the sintered first electrode, wherein the electrolyte is a thin-layerof the solid-oxide electrolyte substance; and forming a second electrodeonto at least part of a surface of the electrolyte, wherein the secondelectrode includes the solid-oxide electrolyte material and a secondelectrochemically active metallic substance.

In still another aspect, the present disclosure provides a process formanufacturing a tubular solid-oxide electrochemical cell, including thesteps of forming an electrode dough that includes a solid-oxideelectrolyte substance, a first electrochemically active metallicsubstance, a first carbon-based pore forming substance, a first binderand a first solvent; extruding the electrode dough in a tubular shape toform a first electrode; and sintering the extruded first electrode. Anelectrolyte slurry including the solid-oxide electrolyte substance and asecond solvent is formed and coated onto at least a part of a surface ofthe sintered first electrode, and the electrolyte coating sintered. Anelectrode slurry including the solid-oxide electrolyte substance, asecond electrochemically active metallic substance, a secondcarbon-based pore forming substance and a third solvent is formed andcoated onto at least a part of a surface of the sintered electrolyte,and the electrode coating is sintered.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the nature, objects, and processesinvolved in this disclosure, reference should be made to the detaileddescription taken in conjunction with the accompanying drawings, inwhich:

FIGS. 1A, 1B and 1C depict a tubular solid-oxide electrochemical cellaccording to one embodiment of the present disclosure; and

FIG. 2 shows a flow chart illustrating one embodiment of a process formanufacturing a tubular solid-oxide electrochemical cell according tothe present disclosure.

DETAILED DESCRIPTION

The present disclosure provides for cost-effective production ofelectrochemical cells for use in electrochemical synthesis that may becustomized and optimized depending on the desired electrochemicalsynthesis reaction. The electrochemical cells of the present disclosurehave a tubular configuration and are made of solid-oxide ceramicmaterials. Tubular solid-oxide electrochemical cells according to thepresent disclosure may be stacked and arranged in variousconfigurations, and have the mechanical and chemical stability anddurability to be used in commercial scale electrochemical synthesis ofvarious gases, such as hydrogen, ammonia, nitric oxide, syngas, andothers.

FIG. 1A shows an isometric view of one embodiment of a tubularsolid-oxide electrochemical cell 100 according to the presentdisclosure. The electrochemical cell 100 is supported by a firstelectrode 110 configured in a tubular shape, upon which an electrolyte120 and a second electrode 130 are disposed. As shown in FIG. 1A, theelectrolyte 120 is disposed as a thin-layer on at least a portion of asurface of the first electrode 110, and the second electrode 130disposed on at least a portion of a surface of the electrolyte 120.

FIG. 1B shows a cross-sectional isometric view through the midsection,and FIG. 1C shows an axial cross-sectional view through part of themidsection, of the tubular solid-oxide electrochemical cell 100 shown inFIG. 1A. The first electrode 110 has an average thickness (T₁) that isgreater than the combined average thickness (T₂) of the thin-layer ofthe electrolyte 120 and the average thickness (T₃) of the secondelectrode 130 taken together. The relatively thick first electrodeprovides mechanical support for the electrochemical cell and allows theelectrolyte to be thinner, which helps reduce both ohmic losses and theamount of energy needed to transport ions through the electrolyte.Typically, the average thickness of the first (support) electrode (T₁)is in a range of about 5 mm to about 50 mm, the average thickness of theelectrolyte (T₂) is in a range of about 5 microns to about 100 microns,and the average thickness of the second electrode is (T₃) is in a rangeof about 5 microns to about 100 microns.

While FIGS. 1A-1C show an embodiment having the electrolyte 120 beingdisposed on an outer surface of the first (support) electrode 110, andthe second electrode 130 being disposed on an outer surface of theelectrolyte 120, the present disclosure includes embodiments where theelectrolyte and second electrode are formed on at least a portion of aninner surface of the first electrode and the electrolyte, respectively,so that the first (support) electrode is the outermost layer of theelectrochemical cell.

Moreover, depending on the desired electrochemical synthesis reaction,the selection of the electrolyte and the configuration of the reactionapparatus, the first (support) electrode may function as the anode andthe second electrode as the cathode, or vice versa (i.e., the first(support) electrode may function as the cathode and the second electrodeas the anode).

The electrolyte used in the present electrochemical cells typically ismade of a solid-oxide electrolyte substance, such as a perovskite, afluorite, and others known in the art. The solid-oxide electrolytesubstance preferably is designed to have high ionic conductivity, lowelectronic conductivity, and high density so as to prevent non-ionizedgaseous reactants from mixing.

Perovskite commonly refers to a class of metal oxide materials having ageneral formula ABO₃, where A refers to a metal cation having arelatively large ionic radius and B refers to a metal cation having arelatively small ionic radius. The crystal structure of perovskitematerials is highly tolerant to vacancy formation and encompassesvarious different phases (such as Aurivilius phase and Ruddleson Popperphase), making perovskite materials well suited for use as ionconducting electrolytes. “A” may include, but is not limited to,monovalent metal cations (M¹⁺, such as Na, K), divalent metal cations(M²⁺, such as Ca, Sr, Ba, Pb), trivalent metal cations (M³⁺, such as Fe,La, Gd, Y) and combinations thereof “B” may include, but is not limitedto, pentavalent metal cations (M⁵⁺, such as Nb, W), tetravalent metalcations (M⁴⁺, such as Ce, Zr, Ti), trivalent metal cations (M³⁺, such asMn, Fe, Co, Ga, Al) and combinations thereof.

Fluorite commonly refers to a class of materials having a face-centeredcubic structure and includes metal oxides having a general formula MO₂,where “M” may include, but is not limited to, divalent metal cations(M²⁺, such as Ca, Sr, Ba, Mg), trivalent metal cations (M³⁺, such as Sc,Y, Yb, Er, Tm, La, Gd, Dy, Sm, Al, Ga, In), tetravalent metal cations(M⁴⁺, such as Ce, Zr, Th, Hf, Bi) and combinations thereof.

Other metal oxide materials that may be used for the solid-oxideelectrolyte substance according to the present disclosure includepyrochlores (having a general formula A₂B₂O₇ or A_(2-x)A′_(x)B₂O₆, whereA is a trivalent metal cation such as Gd, Sm, La, Nd, Eu, Tb, Bi, Y, Dy;A′ is a divalent metal cation such as Ca; and B is a tetravalent metalcation such as Ti, Zr, Ru), brownmillerite (A₂B₂O₅, where A is adivalent cation such as Al, Ca, Sr, Ba; and B is a trivalent metalcation such as Fe, In, Ga, Mn, Cr, Zr, I-If, Ce, Ti) and the like.

Depending on the desired electrochemical synthesis reaction, thesolid-oxide electrolyte substance is selected to be a proton (H⁺)conducting material, or an oxygen ion (O²⁻) conducting material. Thespecific composition of the solid-oxide electrolyte substance andwhether it acts as a proton conductor or an oxygen ion conductor willalso depend on the desired electrochemical synthesis reaction, as thecombination of metal ions (e.g., A, A′, B, or M) may be manipulated tominimize the chemical reactivity of the electrolyte to the reactants andreaction conditions as well as optimize the conductivity of the desiredionic reactant species.

The first and second electrodes used in the present electrochemicalcells typically are made of a mixed ionically/electronically conductivecomposite material of the solid-oxide electrolyte substance and anelectrochemically active substance. Moreover, the first and secondelectrodes preferably are porous and chemically stable in the highlyreducing or oxidizing environment in which the electrochemical cellsoperate. In some embodiments, the first and second electrodes are madeof the same mixed ionically/electronically conductive compositematerial; in other embodiments, the first electrode is made of a firstmixed ionically/electronically conductive composite material and thesecond electrode is made of a second mixed ionically/electronicallyconductive composite materials that is different from the first mixedionically/electronically conductive composite material.

In prior art electrochemical cells, the electrolyte and the electrodestypically have different thermal characteristics so that, upon heating,the electrolyte and the electrodes expand at different rates causing oneor more of the electrodes to crack and/or split away from theelectrolyte. It has been found that, when the electrodes of the presentelectrochemical cells are made of a composite material that incorporatesthe same solid-oxide electrolyte substance used in the electrolyte, itis possible to match the coefficients of thermal expansion (CTEs) of theelectrodes' materials sufficiently with the C′I′E of the electrolyte'smaterials so as to prevent the cracking and/or splitting observed uponheating of prior art electrochemical cells. The mixedionically/electronically conductive composite materials used for theelectrodes typically have at least about 70% by weight of thesolid-oxide electrolyte substance and preferably have a coefficient ofthermal expansion within about ±10% of that of the solid-oxideelectrolyte sub stance.

When the present electrochemical cells are used for electrochemicalsynthesis reactions, the electrodes provide the reaction sites for theoxidation and reduction half-reactions that make up the desiredelectrochemical synthesis reaction. Accordingly, the first and secondelectrodes incorporate an electrochemically active substance, such asmetal or metal oxide or semiconductor substance, that helps catalyze thedesired half-reactions and also provides electronic conductivity to theelectrodes. The electrochemically active substance used will depend onthe desired electrochemical synthesis reaction, and may include, but isnot limited to, Sc, Ti, Zn, Sr, Y, Zr, Au, Bi, Pb, Co, Pt, Ru, Pd, Ni,Cu, Ag, W, Os, Rh, Ir, Cr, Fe, Mo, V, Re, Mn, Nb, Ta, and oxides, alloysand mixtures thereof.

Additionally, the electrodes preferably have a porosity andmicrostructure that allow the reactant gases to migrate throughout theelectrode to come into contact with the electrochemically activemetallic substance, and dissociate and/or react according to the desiredelectrochemical synthesis reaction. The porosity and microstructure ofthe electrodes are provided by incorporating a carbon-based pore formingsubstance which is burned out from the mixed ionically/electronicallyconductive composite material during the formation of the electrodes, aswill be discussed further below. The carbon-based pore-forming substancemay include, but is not limited to, graphite powder, starch, powderedand/or particulate organic polymers (e.g., polymethylmethacrylate,acrylic resin, polyvinylchloride, and the like). The amount and type ofcarbon-based pore forming material may be adjusted to create an optimalporous microstructure for the electrodes, depending on the desiredelectrochemical synthesis reaction.

FIG. 2 shows a flow chart 200 outlining one embodiment of a process formanufacturing a tubular solid-oxide electrochemical cell according tothe present disclosure. In general, the first electrode is formed in atubular shape by extruding an electrode dough, and the extruded firstelectrode is sintered before an electrolyte is coated onto at least aportion of a surface of the first electrode. After the electrolytecoated first electrode is sintered, a second electrode is coated onto atleast a portion of a surface of the electrolyte, and the completedelectrochemical cell is heated to sinter the second electrode. The stepsinvolved in this embodiment will be discussed in more detail below.

The ingredients for the first electrode are combined at 205 and mixedand milled at 210, typically using a solvent in order to adjust theparticle size and particle size distribution of the ingredients. Anysolvent known and used in the art for milling, such as water, may beused. The ingredients for the first electrode include at least asolid-oxide electrolyte substance, a first electrochemically activesubstance, and a carbon-based pore-forming substance. One or more of theother additives discussed below may also be combined, mixed and milledwith these ingredients. After milling to an appropriate particle sizeand distribution, the mixture of ingredients is dried at 220.

A binder and solvent are added at 225 to the mixture of ingredients andthe resulting combination mixed and aged at 230 to form an electrodedough. The binder may be any known aqueous or non-aqueous binder, suchas polyvinyl pyrollidone, polyvinyl alcohol, polyvinyl butyral, anacrylic (such as a polymethacrylate, and the like), a polyacrylamide, apolyethylene oxide, an alginate, a cellulose (such as methylcellulose,ethylcellulose, and the like), a starch, a gum, a styrene or a bindersystem, such as the Duramax™ binders (available from The Dow ChemicalCompany), as well as combinations and mixtures thereof. The solvent maybe any solvent used in ceramic manufacturing, such as water, acetone,ethanol, isopropanol, methyethyl ketone, α-terpineol, and the like, aswell as combinations and mixtures thereof. The selection of binder willdepend on the other electrode ingredients and the solvent, as the bindertypically imparts wet and dry strength to the electrode dough and theresulting electrode and must be compatible with the solvent.

One or more other additives that are known and used in ceramicsmanufacturing also may be added at 225 (if not added previously) toimpart a consistency and other properties that allow the electrode doughto be smoothly extruded. Such additives may include: a dispersant, suchas polyacrylic acid, a phosphoric ester of an alcohol or phenol (e.g.,available under the Beycostat™ mark), and the like; a polymer orpolymerization system, such as acrylamide with a cross linker (e.g.,bis-acrylamide), an initiator (e.g., ammonium persulfate), and acatalyst (e.g., tetramethylethylenediamine); a plasticizer, such asdioctylphthalate and the like; a viscosity modifier; a flocculent; and alubricant. The electrode dough typically has about 30% to about 70%volume of solid ingredients, with the remaining volume being solvent.

After aging, the electrode dough is extruded through a die at 240 toform a first electrode in a tubular shape. Any known extrusion methodmay be used. The tubular first electrode may be made to any length anddiameter, though typically is from about 5 cm to about 150 cm in lengthwith an inner dimension from about 0.5 cm to about 5 cm. The extrudedtubular first electrode generally has a circular cross-section, butother cross-sectional shapes (such as semicircular, oval, square,rectangular, triangular, trapezoidal, star, etc.) are possible andwithin the scope of this present disclosure. For some implementations,depending on the particular synthesis reaction and reactorconfiguration, a non-circular cross-section may provide certainadvantages, such as cell packing, heat dissipation and/or reactant flowthrough or along or around the cell.

The extruded tubular first electrode is dried and sintered at 250.During drying and sintering, the extruded tubular first electrode istypically placed horizontally onto a holder that will prevent the firstelectrode from bending. The extruded tubular first electrode may bedried at room temperature for up to 48 hours, and then sintered to burnoff the additives from the extruded tubular electrode. Sintering mayinvolve gradually heating the extruded tubular electrode, for example,at a rate between about 0.1° C./minute to about 10° C./minute, thoughmore typically between about 1° C./minute to about 2° C./minute, to aset temperature between about 800° C. to about 1600° C. and holding atthe set temperature for up to about 12 hours. In some embodiments,sintering may involve gradually heating in a stepwise manner to one ormore intermediate set temperatures and holding at each intermediate settemperature in order to burn off various different additives. As theextruded tubular electrode is sintered and the carbon-based pore formingsubstance, in particular, burns off, pores are left behind in theelectrode, so that the sintered first electrode is a solid porous cermet(ceramic-metal) composite material.

An electrolyte is formed at 260 by, for example, spray coating, dipcoating or otherwise layering an electrolyte slurry (formed at 255) as athin-layer onto at least a part of a surface of the sintered firstelectrode. The electrolyte may be formed onto a part of an inner surfaceor an outer surface of the sintered first electrode. The electrolyteslurry may be formed by combining the solid-oxide electrolyte substancewith a solvent (such as discussed previously) and optionally one or moreother additives (such as binders, dispersants, polymers, etc. asdiscussed previously) that are commonly used in ceramics manufacturing;mixing and milling; and then adding more solvent to form a slurry havinga consistency to be coated or layered onto the first electrode. Theelectrolyte slurry typically has about 10% to about 30% volume of solidingredients, with the remaining volume being solvent. The electrolyte isthen dried and sintered at 270 to form a solid electrolyte layer.

A second electrode is formed at 280 by, for example, spray coating, dipcoating or otherwise layering an electrode slurry (formed at 275) ontoat least a part of a surface of the sintered electrolyte. The electrodeslurry may be formed by combining the solid-oxide electrolyte substance,a second electrochemically active substance, and a carbon-based poreforming substance with a solvent (such as discussed previously) andoptionally one or more other additives (such as binders, dispersants,polymers, etc. as discussed previously) that are commonly used inceramics manufacturing; mixing and milling; and then adding more solventto form a slurry having a consistency to be coated or layered onto theelectrolyte. The electrode slurry typically has about 10% to about 30%volume of solid ingredients, with the remaining volume being solvent.The second electrode is then dried and sintered at 290, as discussedpreviously, to form a solid porous second electrode.

It should be understood that the operations described herein may beperformed in any order, unless otherwise specified, and embodiments ofthe disclosure may include additional or fewer operations than thosedisclosed herein.

It will be further understood that the articles “a”, “an”, “the” and“said” are intended to mean that there may be one or more of theelements or steps present. The terms “comprising”, “including” and“having” are intended to be inclusive and mean that there may beadditional elements or steps other than those expressly listed.

The foregoing description has been presented for the purpose ofillustrating certain aspects of the present disclosure and is notintended to limit the disclosure. Persons skilled in the relevant artwill appreciate that many additions, modifications, variations andimprovements may be implemented in light of the above teachings andstill fall within the scope of the present disclosure.

We claim:
 1. A process comprising: providing a tubular solid-oxideelectrochemical cell comprising: a porous cathode configured in atubular shape, the porous cathode comprising a first mixedionically/electronically conductive composite material comprising asolid-oxide electrolyte substance having ionic conductivity and a firstelectrochemically active substance having electronic conductivity; anelectrolyte disposed as a thin-layer on at least part of a surface ofthe porous cathode, the electrolyte comprising the solid-oxideelectrolyte substance; and a porous anode disposed on at least part of asurface of the electrolyte, the porous anode comprising a second mixedionically/electronically conductive composite material comprising thesolid-oxide electrolyte substance and a second electrochemically activesubstance having electronic conductivity; connecting the porous cathodeand the porous anode to a power source; and applying a voltage acrossthe porous cathode and the porous anode to perform a catalyticelectrochemical synthesis of ammonia.
 2. The process of claim 1, whereinthe solid-oxide electrolyte substance comprises a H⁺ conductingmaterial.
 3. The process of claim 1, wherein the solid-oxide electrolytesubstance comprises a material selected from the group consisting of aperovskite, a fluorite, a pyrochlore and a brownmillerite.
 4. Theprocess of claim 1, wherein the solid-oxide electrolyte substancecomprises a perovskite having a general formula ABO₃, wherein A isselected from the group consisting of monovalent, divalent and trivalentmetal cations and combinations thereof; and B is selected from the groupconsisting of pentavalent, tetravalent and trivalent metal cations andcombinations thereof.
 5. The process of claim 1, wherein the porouscathode comprises at least about 70% by weight of the solid-oxideelectrolyte substance.
 6. The process of claim 1, wherein the porousanode comprises at least about 70% by weight of the solid-oxideelectrolyte substance.
 7. The process of claim 1, wherein the firstmixed ionically/electronically conductive composite material and thesecond mixed ionically/electronically conductive composite material arethe same.
 8. The process of claim 1, wherein the porous cathode has anaverage thickness in a range of about 5 mm to about 50 mm.
 9. Theprocess of claim 1, wherein the electrolyte has an average thickness ina range of about 5 microns to about 100 microns.
 10. The process ofclaim 1, wherein the porous anode has an average thickness in a range ofabout 5 microns to about 100 microns.
 11. The process of claim 1,wherein a thickness of the porous cathode is in a range of about 5 mm toabout 50 mm, a thickness of the electrolyte is in a range of about 5microns to about 100 microns, and a thickness of the porous anode is ina range of about 5 microns to about 100 microns.
 12. The process ofclaim 1, wherein the first mixed ionically/electronically conductivecomposite material and the second mixed ionically/electronicallyconductive composite material each has a coefficient of thermalexpansion within about 10% of that of the solid-oxide electrolytesubstance.
 13. The process of claim 1, wherein the tubular shape has anouter surface and an inner surface, and the electrolyte and the porousanode are formed on at least a portion of the outer surface.
 14. Theprocess of claim 1, wherein the tubular shape has an outer surface andan inner surface, and the electrolyte and the porous anode are formed onat least a portion of the inner surface.